runtime: don't restore the alternate signal stack on ios

The alternative signal stack doesn't work on ios, so the setup of
the alternative stack was skipped. The corresponding unminitSignals
was effectively a no-op on ios until CL 70130. Skip unminitSignals
on ios to restore the previous behaviour.

For the ios builders.

Change-Id: I5692ca7f5997e6b9d10cc5f2383a5a37c42b133c
Reviewed-on: https://go-review.googlesource.com/70270
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/os_darwin.go

@@ -188,7 +188,11 @@ func minit() {
 // Called from dropm to undo the effect of an minit.
 //go:nosplit
 func unminit() {
-	unminitSignals()
+	// The alternate signal stack is buggy on arm and arm64.
+	// See minit.
+	if GOARCH != "arm" && GOARCH != "arm64" {
+		unminitSignals()
+	}
 }
 
 // Mach IPC, to get at semaphores